In order to obtain image data (magnetic resonance recordings) during magnetic resonance imaging from an area from the inside of the body of an examination object, the body or the body part to be examined is exposed to a static main magnetic field (e.g., referred to as B0 field) that is as homogeneous as possible. As a result, the macroscopic magnetization within the body is aligned parallel to the direction of the B0 field, e.g., the z direction. In addition, radio-frequency antennas are used to radiate radio-frequency pulses into the examination object, the frequency of which being in the range of the resonant frequency, what is known as Larmor frequency, of the nuclei to be excited (e.g., hydrogen nuclei) in the present magnetic field. Therefore, the radio-frequency pulses will be referred to below as magnetic resonance radio-frequency pulses. The magnetic flux density of the radio-frequency pulses may be designated as B1 (and the transmitted RF signals therefore also as B1 field, in short). With the aid of the radio-frequency pulses, the macroscopic magnetization within the examination object is excited such that it is deflected out of its equilibrium position parallel to the main magnetic field B0 by what is known as a “flip angle.” The macroscopic magnetization then proceeds about the z-direction and relaxes gradually. The in-phase movement of the microscopic spin about the precession cone may be considered to be macroscopic nucleus magnetization in the x/y plane (perpendicular to the z-direction). The magnetic resonance measurement signals generated during the relaxation of the nucleus magnetization are recorded as what is called “raw data” using radio-frequency receiver antennas. The magnetic resonance images of the examination object are finally reconstructed on the basis of the acquired raw data, where spatial encoding is carried out with the aid of rapidly switched gradient magnetic fields that are superposed on the main magnetic field during the transmission of the magnetic resonance radio-frequency pulses and/or the acquisition of the raw data.
The magnetic resonance raw data are recorded in most cases with the aid of the local coils, which are positioned directly on, at, or under the examination object, e.g., the patient or subject. This has, among others, the advantage that the signal-to-noise ratio may be significantly increased and that a better image quality may be achieved. In some cases, local coils are also used for transmitting the B1 field. Especially when recordings of a relatively large spatial region are intended to be made, a large number of such local coils are used. In many examinations, for example, the patient is completely covered by individual local coils, for example, head coils, chest coils, stomach coils, leg coils, etc.
For transmitting signals between the local coils and the local coil communication interface arranged fixedly in the magnetic resonance system, cables may be used. In the following text, a local coil communication interface is understood to describe any desired arrangement of magnetic resonance transmitting and/or receiving devices, e.g., corresponding interfaces for transmitting the B1 field and for receiving the raw data, and of control devices for the local coils for transmitting control signals to the local coils, for example, for switching from transmitting operation to receiving operation, for detuning, etc., and/or for receiving status signals therefrom. In the process, all these components may be integrated in a common device, or they may be spatially separate devices.
The cables have a relatively thick electrical insulation to protect the patient against excessive heating, such as for transmitting the radio-frequency signals. In addition, the radio-frequency cables have, at relatively short spacings (for example, every 30 cm in the case of a 3 Tesla lamp magnetic resonance system), relatively voluminous sheath wave barriers in order to prevent radio-frequency currents from passing over the cable shielding. This may in turn lead to interference with the B1 field. Another problem lies in the fact that the local coils may have to be coupled via relatively expensive multiway radio-frequency connectors to the corresponding devices of the local coil communication system of the magnetic resonance system. The radio-frequency connectors are additionally constructed such that they are protected against contamination from various liquids, as may occur during clinical operation, by way of suitable mechanical seals, etc., in particular if no cable is inserted. Another disadvantage of the cabling is that the preparation of a patient for examination is time-consuming on account of the relatively high effort in positioning the local coils, the subsequent fixing, and subsequently the necessary cabling.
In order to reduce the outlay, it has already been suggested to transmit various signals (and, in some embodiments, the measured raw data), wirelessly from the local coils. However, since the number of local coils used or of their channels continuously increases with the improvement of the examination capabilities (currently, receiving coils with 32 channels or even 64 channels are already used as a matter of routine, for example), the required bandwidth for the signal transmission also increases correspondingly, and may even exceed the currently still freely available signal bandwidth. Such radio transmission systems additionally have the disadvantage that a redundant transmission via a plurality of channels may be necessary, since multiple reflections at the metallic structures within the measurement space of the scanner may take place.
In U.S. Publication No. 2012/0161768 A1, a solution was additionally suggested, in which the flexible microstrip conductors are incorporated within a blanket or item of clothing, which are connected to the local coils with the signals being transmitted via the microstrip conductors (stripline). However, the microstrip conductors are constructed as electrical conductors, such that transmission both of radio-frequency signals and of DC current is possible. Even with such conductors, provisions similar to sheath wave barriers are therefore taken such that no interference fields may occur, in particular in the kHz range, the frequency range of the gradient coils.